Calibration for blood pressure pulses

ABSTRACT

Disclosed is an improved arrangement for allowing more accurate automatic calibration for a system in which blood pressure pulses are derived for remote sites from a calibrated measurement and a waveform measurement, when these are made at different sites. Where a calibrated measurement is made, say, at the brachial artery (11), and a waveform recorded at the radial artery (12), a waveform is derived for the calibrated site using a known transfer function, this drived waveform is then calibrated by the measurements already made, and the calibrated waveform for any remote site, for example the ascending aorta (14), can be derived using a known transfer function.

TECHNICAL FIELD

The present invention relates to the measurement of peripheral bloodpressure pulse waveforms and the use of these waveforms to determinepulse waveforms at other sites. More particular, it is concerned withthe calibration of these waveforms.

BACKGROUND ART

A technique for utilising peripheral pressure pulses to determine thepulse waveform at other body sites, particularly the ascending aorticwaveform, is disclosed in U.S. Pat. No. 5,265,011 by Michael F.O'Rourke. The books "The Arterial Pulse" by O'Rourke, Kelly and Avoliopublished by Lea Febiger, Philadelphia USA 1992 and "ArterialVasodilation", by O'Rourke, Saffer, and Dzau, published by Arnold,London 1993 disclose the use of a measured peripheral waveform todetermine the waveform at another site, using an empirically determinedtransfer function. The derived waveform can be calibrated, as it isderived from the peripheral waveform which itself is calibrated. Inthese documents, it is assumed that the brachial cuff sphygmomanometermeasurement can be used to directly calibrate waveforms measured at theradial artery. It is assumed that relatively little change occurs in theparameters between the brachial artery and the radial artery. Whilstthis is a reasonable working assumption, it is not strictly accurate.

It is an object of the present invention to provide an improvedcalibration procedure, such that derived waveforms are able to be moreaccurately calibrated.

SUMMARY OF INVENTION

According to a first aspect, the present invention provides a method ofcalibrating a derived pressure pulse waveform, said derived waveformbeing determined by processing a peripherally measured waveform,comprising the steps of:

1. measuring at a first site a blood pressure pulse waveform;

2. measuring substantially simultaneously using a calibrated instrumentthe systolic and diastolic pressures at a second site;

3. determining the pulse waveform shape at said second site from thewaveform measured at said first site using a first predeterminedtransfer function;

4. calibrating the waveform at said second site using the measuredsystolic and diastolic pressures;

5. determining the pulse waveform shape at a third site directly orindirectly from the waveform measured at said first site using a secondpredetermined transfer function;

6. calibrating the waveform at said third site directly or indirectlyfrom the calibrated waveform from said second site.

Preferably, the measurements at the first and second sites are performednon-invasively. The first site may be, conveniently, the radial arteryor the finger, and the second site may be the brachial artery, althoughany convenient site may be chosen.

It will be appreciated that the present invention is not limited to thesteps being performed in the particular order shown above. The threedifferent transfer functions mentioned above may in fact be only twodifferent transfer functions, depending upon how the calculation isdesired to be performed. The user is only required, in any case, tomeasure the waveform, and make a calibration measurement. For example,the measured waveform at the first site may be used to derive anuncalibrated waveform at the second site, which is then calibrated bymeasurement at that site. The calibrated waveform may then be used tocalibrate the waveform at the first site. From the calibrated secondsite waveform, a calibrated waveform at the third site can be directlyobtained by applying the appropriate transfer function.

It will be understood that the calculation process may actually beperformed in a number of equivalent ways. However, the general principleof the present invention is retained--that is, that a measurement at onesite is used to provide calibration for the derived waveform, and at asecond site a waveform is acquired for calculating the derived waveform,with different transfer functions being used between the first andsecond sites to the site for which the derived waveform is required.

The measurement steps may be performed with manually operatedinstruments, and the output data processed by any suitable processor, orthe waveform may be measured continuously and calibrated periodically orin real time, either manually or automatically.

This approach allows for a convenient waveform measurement site to beused, which may not be reliable or convenient in terms of accuracy ofcalibrated pressure measurements, with calibration performed at aconvenient site for calibrated measurement.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will now be described with reference to theaccompanying figures, in which:

FIG. 1 is a schematic diagram illustrating the sites where measured andderived pressures are located;

FIG. 2 is a flow chart illustrating the determination of the remote sitepressure waveform;

FIG. 3 is an illustration showing typical pressure waveforms for theascending aorta, brachial artery and radial artery; and

FIGS. 4 & 5 are graphs showing the transfer functions between thebrachial artery and the ascending aorta, and between the radial arteryand the ascending aorta.

DESCRIPTION OF EMBODIMENT

It will be appreciated that while the present invention is describedwith reference to particular sites for pressure measurement and forwhich pressures are derived, the inventive principle is equallyapplicable to other sites. Moreover, whilst the use of non-invasivetechniques is presently preferred for practical reasons, the presentinvention contemplates the use of an invasively derived waveform orcalibration if available or desired. The inventive method may beincorporated in software in any suitable digital processing device, ofthe type disclosed in the references cited above, as would be readilyunderstood by those skilled in the art.

Referring to FIG. 1, this is a schematic illustration of the pressuremeasurement sites used according to the following discussion. Threesites are of relevance to this discussion on person 10. The site forwhich the calibrated pressure waveform is required is illustratively theascending aorta 14. It will be appreciated that according to the presentinvention this could be any other site for which suitable transferfunctions have been derived, for example, the carotid artery 15. Thecalibrated measurement may be derived by conventional sphygmomanometryat the brachial artery 11. The waveform may be acquired for the purposesof this example at the radial artery 12--however, it will be appreciatedthat other sites may be used, for example the finger 13, or carotidartery 15. An uncalibrated waveform may even be acquired at the site forwhich a calibrated waveform is required, if this is accessible to thepressure measurement instrument.

The waveform may be acquired using any suitable means--for example, atonometer, or at the finger using the FINIPRES device, or by anin-dwelling pressure catheter. Such devices are widely availablecommercially, and the reader will be familiar with their operation. Theprocess of acquiring a waveform using a tonometer is described in theO'Rourke et al references cited above. Similarly, the general principlesof deriving a waveform for the ascending aorta using transfer functions,and the techniques for deriving such transfer functions, are describedin those documents, which are hereby incorporated by reference. Varioustransfer functions have been published in the scientific literature,which could be adopted for use in the present invention. It will beappreciated that the transfer functions may be derived on a differentbasis, if desired.

FIG. 2 illustrates the process according to the present invention. Thewaveform is measured at, for example, the radial artery, and the knowntransfer function used to derive the shape of the waveform at theascending aorta. A suitable transfer function is shown in FIGS. 4 & 5between each of the brachial and radial artery, and the ascending aorta.FIG. 3 illustrates typical pressure waveforms at the radial artery,brachial artery and ascending aorta.

At the same time, or shortly before or after, conventionalsphygmomanometry may be used to measure the systolic and diastolicpressures at a suitable site, for example at the brachial artery. Thetransfer function defines a relationship between the relative waveformmagnitudes at the different sites related by the function. From theradial artery waveform, a transfer function can be used to derive anuncalibrated waveform at the brachial artery, which can then becalibrated using the measured systolic and diastolic pressures at thatsite. The ascending aorta waveform can then be calibrated from therelative magnitude of the waveforms at the ascending aorta and thebrachial artery, and the known calibration at the brachial artery.Accordingly, the derived ascending aorta pressure waveform can becalibrated. It will be appreciated that the processing using transferfunctions is conveniently carried out using the Fourier transform of thewaveforms, using a suitably programmed microprocessor.

It will be appreciated that an alternative calculation process could beused. For example, the transfer function between the brachial artery andthe radial artery could be used to determine a calibrated radialpressure waveform, and the transfer function to the ascending aorta usedto determine the calibrated ascending aorta waveform from the radialwaveform.

Another alternative would be to use a transfer function from thecalibrated brachial artery waveform to derive the calibrated ascendingaorta pressure waveform using the appropriate transfer function. Thecalculation could also be performed in real time, subject to a suitableprocessor and software being employed, as would be apparent to thoseskilled in the art. The basic technique remains the same.

It will be appreciated that the present technique allows for acalibrated waveform to be derived for any selected site, from a waveformmeasured at a different site, provided the corresponding transferfunction is known.

The reader will appreciate that variations and additions are possiblewithin the spirit and scope of the invention, within the generalinventive concept.

I claim:
 1. A method of calibrating a derived pressure pulse waveform,said derived waveform being determined by processing a peripherallymeasured waveform, comprising the steps of:a. measuring at a first sitea blood pressure pulse waveform; b. measuring substantiallysimultaneously using a calibrated instrument the systolic and diastolicpressures at a second site; c. determining the pulse waveform shape atsaid second site from the waveform measured at said first site using afirst predetermined transfer function; d. calibrating the pulse waveformshape at said second site using the measured systolic and diastolicpressures to provide a calibrated waveform; e. determining the pulsewaveform shape at a third site directly or indirectly from the waveformmeasured at said first site using a second predetermined transferfunction; f. calibrating the waveform at said third site from thecalibrated waveform from said second site, so that a calibrated waveformis derived from said third site.
 2. A method according to claim 1,wherein said step f. includes deriving the calibrated waveform at saidthird site by applying said second transfer function to the calibratedwaveform at said second site.
 3. A method according to claim 2, whereinsteps c. to f. are performed by software.
 4. A method according to claim2, wherein the first site is selected from the group comprising theradial artery and the finger, and the second site is the brachialartery.
 5. A method according to claim 1, wherein said step e. includescalibrating the measured waveform at said first site from the calibratedwaveform at said second site, and deriving said third site waveform byapplying a said second transfer function from said calibrated waveformat said first site to said third site.
 6. A method according to claim 1,wherein the first site is selected from the group comprising the radialartery and the finger, and the second site is the brachial artery.
 7. Amethod according to claim 1, wherein steps 3 to 6 are performed bysoftware.
 8. A method according to claim 5, wherein steps c. to f. areperformed by software.
 9. A method according to claim 5, wherein thefirst site is selected from the group comprising the radial artery andthe finger, and the second site is the brachial artery.
 10. A methodaccording to claim 1, wherein the first site is the same as the thirdsite.
 11. A method according to claim 1, wherein said step f. includesderiving the calibrated waveform at said third site by applying a thirdtransfer function to the calibrated waveform from said second site. 12.A method according to claim 1, wherein said step f. includes derivingthe calibrated waveform at said third site by applying a third transferfunction to the waveform measured at said first site.
 13. A methodaccording to claim 1, wherein said step f. includes deriving thecalibrated waveform at said third site by applying a third transferfunction to the pulse waveform shape from said second site.